The present invention is directed to an electrode for piezoelectric composites. More specifically, the present invention is directed to an electrode for piezoelectric composites having acoustic attenuation as well as to a method for the manufacture thereof.
Piezoelectric transducers for ultra-sound transmitter/receiver equipment are known. For example, European Patent Application EP 015 4706 discloses a piezoelectric transducer for ultra-sound transmitter/receiver equipment.
One of the difficulties in making piezoelectric transducers is the problem of the intrinsic impedance of piezo-ceramic that differs extremely relative to air and of the difficulty, in matching the acoustics of the piezo-ceramic to the air that is connected therewith. The European patent application solves these problems by use of a sandwich structure. Lamellae or foils composed of piezoelectric ceramic material as well as laminae or foils of a dimensionally stable material that is not piezoelectric are stacked on top of one another in an alternate arrangement. These lamellae or foils are firmly joined to one another by, for example, gluing to form a one-piece member. The union of the materials is so secure that a subsequent shaping processing can still be carried out, for example by grinding or other erosion of a surface of this member that is usually cuboid. When the wavelength of the operating frequency of the transducer is long in comparison to the structural dimensions thereof or, respectively, relative to the distances between the individual lamellae or foils, then this is referred to as a composite.
In constructing these transducers, the dimensionally stable material located between the individual lamellae should be chosen so as to exhibit a value of intrinsic impedance that is many times lower than that of the ceramic. Such dimensionally stable materials include: polyurethane foam; silicone caoutchouc (rubber); polyethylene; polystyrol foam; and similar plastics.
These transducers also include electrodes. Thin metal layers of, for example, nickel or silver, that are sputtered on, are usually used as an electrode material for such sandwich transducers. Because of the high mechanical stressing they are subjected to, these electrodes represent the weak point of the overall transducer structure.
The electrodes must exhibit adequate strength so that they do not tear off given great mechanical distortion (for example, due to thermal expansion of the plastic). In order to achieve a sufficient strength, the thickness of the electrode layer on the ceramic lamellae must be appropriately reinforced. But, reinforcing the thickness of the electrode layer results in a reduction of the acoustic attenuation of the individual transducer elements (ceramic plus electrodes). An acoustic crosstalk between the transducer elements occurs, whereby the electrical or, respectively, acoustic signals (when receiving or, respectively, transmitting sound) are overlayed and distorted. An adequately great acoustic decoupling is thereby no longer possible.
The present invention overcomes the disadvantages of the prior electrodes and transducers by providing an electrode for the ceramic lamellae or, respectively, foils of piezoelectric composites that exhibits a high acoustic attenuation and is adequately elastic so that it does not tear off given the high mechanical distortions that occur.